Indazole-type alkaloids from Nigella sativa seeds exhibit antihyperglycemic effects via AMPK activation in vitro.

Six rare naturally occurring indazole-type alkaloids including two new compounds, 17-O-(β-d-glucopyranosyl)-4-O-methylnigellidine (1) and nigelanoid (2), and four known compounds (3-6) were isolated from a defatted extract of Nigella sativa (black cumin) seeds. 17-O-(β-d-Glucopyranosyl)-4-O-methylnigellidine (1) increased glucose consumption by liver hepatocytes (HepG2 cells) through activation of AMP-activated protein kinase (AMPK). Also, this is the first report of compounds 4 and 6 from a natural source.

AMP-activated protein kinase (AMPK) is an enzyme that plays a key role in cellular energy homeostasis, and the AMPK pathway performs a central function in the regulation of glucose and lipid metabolism. The activation of AMPK can stimulate hepatic fatty acid oxidation and ketogenesis, inhibit cholesterol synthesis, lipogenesis, and triglyceride synthesis, stimulate skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulate insulin secretion by pancreatic beta-cells.[1] Moreover, AMPK has been shown to be a target for antidiabetic drugs, including metformin, and several plant natural products derived from traditional medicines.[1]

Nigella sativa Linn. (Ranunculaceae), commonly known as black cumin, grows in Mediterranean and Middle Eastern countries, South Europe, and Southwest Asia.[2] The seeds of N. sativa have been consumed for centuries and are widely used as a spice and traditional medicine for the treatment of various ailments including diabetes.[3] Previous phytochemical investigations of N. sativa seeds has led to the identification of oils, saponins, flavonoids, and alkaloids.[4−8] Notably, to date, N. sativa is one of only two Nigella species reported among all natural sources to contain indazole-type alkaloids.[8]

The seeds of N. sativa have been widely studied for their antidiabetic effects,[9] and its most abundant oil constituent, thymoquinone, is implicated as a major bioactive compound responsible for this activity.[10] However, defatted and aqueous extracts of N. sativa seeds, from which the aforementioned rare indazole-type alkaloids were isolated,[8] have also been reported to show antidiabetic effects.[11] Furthermore, the in vivo antidiabetic activity of a N. sativa seed extract was reported to be mediated through the AMPK pathway,[12] but whether indazole-type alkaloids contribute to these effects is not known. Therefore, a defatted extract of N. sativa seeds was investigated to identify bioactive antidiabetic compounds targeting the AMPK pathway. Herein, the isolation and structure elucidation of six rare, naturally occurring indazole-type alkaloids, including two new (1 and 2) and four known (3–6) alkaloids, are reported.

Compound 1, a yellow, amorphous solid, displayed a molecular formula of C25H31N2O7, as determined by 13C NMR data and an HRESIMS ion at m/z 471.2129 [M]+ (calcd for C25H31N2O7, 471.2126) with 12 indices of hydrogen deficiency. In the 1H NMR data (Table 1), an AA′BB′ spin system with signals at δH 7.59 (d, J = 8.3 Hz, H-15, 19) and 7.32 (d, J = 8.3 Hz, H-16, 18), two aromatic protons at δH 7.17 (brs, H-7) and 6.75 (brs, H-5), and four methylene signals at δH 4.55 (t, J = 6.4 Hz, H-10), 4.43 (t, J = 6.0 Hz, H-13), 2.34 (m, H-11), and 2.21 (m, H-12) were observed, as well as a methyl signal and a methoxy signal at δH 2.59 (3H, s) and 3.81 (3H, s), respectively. The 13C NMR (Table 1) and HSQC data revealed the presence of 25 carbon resonances, comprising two methyl, five methylene, 11 methine (six sp2 and five sp3), and three quaternary carbons (C-3, C-6, and C-14), an N-containing tertiary carbon (C-8), an N,N-disubstituted secondary carbon (C-2), and two oxygenated tertiary carbons (C-4 and C-17). The 1H NMR data of 1 also showed the presence of a β-glucopyranose moiety, the anomeric proton of which resonated at δH 5.07 (1H, d, J = 7.2 Hz, H-1′). The aforementioned physical data suggested that compound 1 was likely an indazole-type alkaloid, which was supported by the fact that these naturally occurring compounds have been observed only in this genus.[8]

Table 1

1H NMR and 13C NMR Data of Compounds 1 and 2a

	1		2
no.	δC	δH (J in Hz)	δC	δH (J in Hz)
2	143.2		147.5
3	109.9		114.7
4	154.8		189.5
5	105.6	6.75 brs	51.9	2.86 d (16.4)
				2.68 d (16.4)
6	147.6		71.3
7	101.0	7.17 brs	34.2	3.19 (2H) s
8	142.6		152.6
10	46.6	4.55 t (6.4)	47.2	4.51 m
				4.43 m
11	19.3	2.34 (2H) m	18.8	2.27 (2H) m
12	19.8	2.21 (2H) m	19.2	2.17 (2H) m
13	48.4	4.43 t (6.0)	48.3	4.34 t (6.0)
14	118.7		113.8
15	131.8	7.59 d (8.3)	131.7	7.46 d (8.7)
16	116.2	7.32 d (8.3)	115.3	6.97 d (8.7)
17	159.8		160.7
18	116.2	7.32 d (8.3)	115.3	6.97 d (8.7)
19	131.8	7.59 d (8.3)	131.7	7.46 d (8.7)
20	21.6	2.59 (3H) s	28.1	1.53 (3H) s
OCH3	55.0	3.81 (3H) s
1′	100.4	5.07 d (7.2)
2′	73.4	3.51 m
3′	77.0	3.52 m
4′	70.0	3.42 t (9.1)
5′	76.6	3.51 m
6′	61.1	3.94 dd (12.0, 2.0)
		3.72 dd (12.0, 5.8)

Data were measured in methanol-d4 at 500 MHz (1H) and 125 MHz (13C).

Analysis of the 2D NMR (including 1H–1H COSY, HSQC, and HMBC) data permitted the construction of the structure of compound 1. After the assignment of all the protons to their bonding carbons by the HSQC data, a hexose moiety (C-1′ to C-6′) and a subunit (C-10 to C-13) (drawn with bold bonds in Figure 1) were established by the 1H–1H COSY data. The HMBC correlation (Figure 1) from H-1′ to C-17 (δC 159.8) assigned the sugar moiety at C-17 of a p-substituted benzene ring (ring A). A tetrasubstituted benzene ring (ring D) was determined by the HMBC correlations from H3-20 to C-5 (δC 105.6), C-6 (δC 147.6), and C-7 (δC 101.0), from H-5 to C-3 (δC 109.9) and C-4 (δC 154.8), and from H-7 to C-3 and C-8 (δC 142.6), which also allowed for the attachment of Me-20 to C-6. The HMBC correlation from the methoxy protons (δH 3.81) to C-4 indicated that a methoxy group was linked to C-4. An sp2 quaternary carbon, C-2 (δC 143.2), was determined to be attached to C-3 and C-14 (δC 118.7) based on the HMBC correlation from H-15 to C-2 and the long-range HMBC (J4) correlation from H-5 to C-2. The chemical shifts of two methylene groups (C-10, δC 46.6; C-13, δC 48.4) implied that they were linked to nitrogen. The HMBC correlations from H-10 to C-8 and from H-13 to C-2 suggested that CH2-10 and CH2-13 were connected to C-8 and C-2 via nitrogen, respectively. The chemical shift of C-2 (δC 143.2) required that it was connected to nitrogen to form an azomethine moiety. Finally, it was apparent that the two nitrogens, N-1 and N-9, were connected based on the requirement of the molecular formula and index of hydrogen deficiency. The d-configuration of the glucopyranosyl moiety was determined by acid hydrolysis. The released glucose was identified by co-TLC and comparison of optical rotation with an authentic sample. Thus, the structure of compound 1 was elucidated as 17-O-(β-d-glucopyranosyl)-4-O-methylnigellidine. Compound 1 is the first glucosylated indazole-type alkaloid isolated from a natural source.

Figure 1

Key 1H–1H COSY (—) and selected HMBC correlations (H → C) of compounds 1 and 2.

Compound 2 (nigelanoid) was obtained as a colorless, amorphous powder with a molecular formula of C18H21N2O3, as determined by 13C NMR and an HRESIMS ion at m/z 313.1554 [M]+ (calcd for C18H21N2O3, 313.1547). Analysis of the 1D and 2D NMR data revealed that compound 2 has a similar chemical structure to compound 1. The striking difference was the absence of the sugar moiety and the two aromatic proton signals in the 1H NMR spectra (Table 1) of compound 2, compared to 1. In the 1H NMR spectrum, an AA′BB′ spin system at δH 7.46 (2H, d, J = 8.7 Hz, H-15, 19) and 6.97 (2H, d, J = 8.7 Hz, H-16, 18) and four methylene signals at δH 4.51 (1H, m, H-10), 4.43 (1H, m, H-10), 4.34 (2H, t, J = 6.0 Hz, H-13), 2.27 (2H, m, H-11), and 2.17 (2H, m, H-12) indicated the presence of rings A (p-substituted benzene ring) and B. The HMBC correlations (Figure 1) from H3-20 (δH 1.53) to C-5 (δC 51.9), C-6 (δC 71.3, oxygenated tertiary carbon), and C-7 (δC 34.2) indicated a consecutive linkage of C-5, C-6, and C-7 and also allowed for the attachment of Me-20 to C-6. A hydroxy group was assigned to C-6 based on the chemical shift of C-6 (δC 71.3). The HMBC correlations from H-5 to C-3 (δC 114.7) and C-4 (δC 189.5) and from H-7 to C-3 and C-8 (δC 152.6) enabled the construction of a six-membered D-ring with an α,β-unsaturated carbonyl moiety. Similarly, the HMBC correlations from H-10 to C-8 and from H-13 to C-2 (δC 147.5) were observed. Thus, the planar structure of compound 2 was established. The assignment of the 6S absolute configuration was based on the electronic circular dichroism (ECD) data displaying a negative Cotton effect at 281 nm (n → π* transition) related to the helicity rule of the cyclohexenone ring carbonyl.[13] The structure of compound 2 was thus elucidated as depicted and was assigned the trivial name nigelanoid.

Four known indazole-containing alkaloids were also isolated and identified as nigellidine (3),[8b] 4-O-methylnigellidine (4),[8b] nigeglanine (5),[14] and 4-O-methylnigeglanine (6)[15] on the basis of their NMR and ESIMS data. Notably, compounds 4 and 6 have been previously obtained through synthesis[8b,15] and are being reported here for the first time from a natural source.

Because N. sativa seeds have been traditionally used as a treatment for diabetes,[16] the antihyperglycemic abilities of the compounds were evaluated in HepG2 hepatocytes by measuring glucose levels in the media and their effects on AMPK phosphorylation. None of the isolates were cytotoxic to the cells at concentrations reaching 100 μM (see Supporting Information), and therefore they were evaluated at their lowest test concentration (of 25 μM) along with the clinical antidiabetic drug metformin (at 1 mM) to investigate their abilities to regulate glucose consumption in HepG2 cells. All of the isolates significantly decreased glucose levels in the cell supernatants by 8–29%, compared to the control, while metformin decreased glucose levels by 24% (Figure 2A). Notably, 17-O-(β-d-glucopyranosyl)-4-O-methylnigellidine (1) showed the best ability to increase glucose consumption, as evidenced by the fact that it decreased glucose levels by 29% in HepG2 cell culture media. Compound 1 is the only isolate that contains a sugar moiety, and on the basis of this preliminary structure–activity relationship (SAR) observation, it is possible that this structural feature imparts increased glucose consumption ability to this type of compound. However, further SAR studies would be required to confirm this.

Figure 2

(A) Effect of the isolates (at 25 μM) on glucose consumption in HepG2 cells. HepG2 cells were acclimated to a low-glucose DMEM overnight and then treated with the isolates or metformin in fresh media. Media was collected 7 h later, and glucose concentration was determined. The values are expressed as the means ± SD of three individual samples. *p < 0.05 as compared to the solvent control (0.1% DMSO). (B) AMPK phosphorylation status in HepG2 cells. HepG2 cells were incubated with metformin (Met) and compounds 1 and 6 (each at 25 and 100 μM, respectively) for 24 h, and then cell lysates were collected. Relative p-AMPK and total AMPK were measured by Western blot.

To further elucidate whether the isolates increase glucose consumption via AMPK activation, the effects of compounds 1 and 6 on phosphorylation of AMPK level in HepG2 cells were examined. As shown in Figure 2B, Western blot analysis showed that compounds 1 and 6 (at 25 and 100 μM concentrations) increased AMPK phosphorylation (Thr172) in a concentration-dependent manner. Thus, our preliminary in vitro studies support the published animal study wherein the antidiabetic activity of an N. sativa seed extract was reported to be mediated through the AMPK pathway.[12]

In summary, previous to this study, only four indazole-type alkaloids had been reported from nature (all obtained from the Nigella genus[17]), but none of these compounds were evaluated for their biological effects with regard to the antidiabetic properties attributed to N. sativa seeds. In the current study, six indazole-type alkaloids (1–6) were isolated and identified from the defatted extract of N. sativa seeds. Among them, 17-O-(β-d-glucopyranosyl)-4-O-methylnigellidine (1) and nigelanoid (2) are new compounds, and compounds 4 and 6 are being reported from a natural source for the first time. In addition, 17-O-(β-d-glucopyranosyl)-4-O-methylnigellidine (1) showed more potent ability to regulate glucose consumption than metformin, which was mediated by activation of AMPK. These studies suggest that naturally occurring indazole-type alkaloids could contribute toward the antidiabetic properties reported for N. sativa seeds. Also, the compounds identified herein could serve as lead scaffolds for the synthesis of structural analogues with more potent antihyperglycemic activities.

Experimental Section

General Experimental Procedures

Optical rotations were measured on an Auto Pol III automatic polarimeter (Rudolph Research, Flanders, NJ, USA) at room temperature. The IR spectra were recorded on a Nicolet 380 FT-IR spectrometer. The UV spectra were measured on a Shimadzu UV-2550 UV–visible spectrophotometer. 1D and 2D NMR data were recorded on a Varian 500 MHz instrument with TMS as internal standard. HRESIMS data were acquired using a synapt G2-S QTOF mass spectrometer (Waters, Milford, MA, USA). Semipreparative HPLC separations were performed on a Hitachi Elite LaChrom system consisting of an L2130 pump, an L-2200 autosampler, an L-2455 diode array detector, and a Phenomenex Luna C18 column (250 × 10 mm, S-5 μm), all operated by EZChrom Elite software. All solvents were of ACS or HPLC grade and were obtained from Sigma-Aldrich (St. Louis, MO, USA) through Wilkem Scientific (Pawcatuck, RI, USA). Silica gel (230–400 mesh, Sorbent Technologies), Sephadex LH-20 gel (Amersham Biosciences), and MCI gel (CHP20P, 63–150 μM, M & M Industries Inc.) were used for column chromatography. Standards of d-glucose and metformin were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Plant Material

N. sativa seeds were collected and authenticated by matching the macroscopic and microscopic characteristics to internal monograph and botanical reference standards by Verdure Sciences (Noblesville, IN, USA). A voucher specimen (VS-NSSP-001) has been deposited in the Heber-Youngken Herbarium and Greenhouse, College of Pharmacy, University of Rhode Island.

Extraction and Isolation

The air-dried, ground powder of N. sativa seeds (2.0 kg) was extracted with n-hexane (4 L × 3) by maceration at room temperature (3 days each time) to afford 298.5 g of crude n-hexane extract. The residue was dried and extracted with MeOH (4 L × 3) by maceration at room temperature (3 days each time) to yield 123.1 g of MeOH extract. To further defat this extract, a portion (122.0 g) was reconstituted in MeOH (500 mL) and partitioned with n-hexane (500 mL × 3) to yield n-hexane (12.3 g) and MeOH (110.0 g) extracts, respectively. The MeOH extract (108.0 g) was chromatographed over a column of MCI gel (MeOH–H2O, 50:50 to 100:0, v/v) to yield five fractions (A–E). Fraction B (5.2 g) was subjected to separation on a Sephadex LH-20 column (3 × 70 cm), eluted with isocratic MeOH, to obtain three fractions, B1–B3. Fraction B2 (1.3 g) was separated by silica gel chromatography (CC) (4.5 × 40 cm) eluted with a gradient of CHCl3–MeOH (10:1 to 1:1 v/v) to obtain six subfractions, B2a–B2f. Purification of subfraction B2c (194.4 mg) by semipreparative HPLC, eluting with MeOH–H2O (0–16 min: 10:90 to 41:59; 16–17 min: 41:59 to 100:0; 17–18 min: 100:0; 18–19 min: 100:0 to 10:90; 19–26 min: 10:90; v/v, 3 mL/min), yielded nigeglanine (5) (1.8 mg). Fraction D (3.5 g) was purified by semipreparative HPLC by eluting with MeOH–H2O (0–17 min: 20:80 to 55:45; 17–18 min: 55:45 to 100:0; 18–20 min: 100:0; 20–21 min: 100:0 to 20:80; 21–28 min: 20:80; v/v, 3 mL/min) to yield compounds 1 (9.8 mg), 2 (2.0 mg), nigellidine (3) (3.6 mg), 4-O-methylnigellidine (4) (7.4 mg), and 4-O-methylnigeglanine (6) (12.6 mg).

17-O-(β-d-Glucopyranosyl)-4-O-methylnigellidine (1):

yellow, amorphous solid; [α]20D −11 (c 0.002, MeOH); UV (MeOH) λmax (log ε) 330 (4.52), 278 (4.39), 228 (3.91) nm; IR νmax 3345, 1618, 1581, 1456, 1382, 860 cm–1; 1H NMR and 13C NMR data, see Table 1; HRESIMS m/z 471.2129 [M]+ (calcd for C25H31N2O7, 471.2126).

Nigelanoid (2):

colorless, amorphous powder; [α]20D −14 (c 0.008, MeOH); UV (MeOH) λmax (log ε) 285 (4.28), 226 (3.95) nm; IR νmax 3415, 1678, 1613, 1598, 1385, 878 cm–1; 1H NMR and 13C NMR data, see Table 1; HRESIMS m/z 313.1554 [M]+ (calcd for C18H21N2O3, 313.1547).

Acid Hydrolysis of Compound 1 and Sugar Analysis

Compound 1 (2 mg) was added to a mixture of concentrated HCl (0.5 mL), H2O (1.5 mL), and dioxane (3 mL) and refluxed for 2 h. After completion of the reaction (monitored by TLC), H2O was added to the reaction mixture, which was extracted with CHCl3 (3 × 5 mL). The aqueous layer was neutralized with NaHCO3, concentrated to dryness under reduced pressure, and purified by Sephadex LH-20 chromatography to give a sugar fraction. The sugar fraction was determined to be d-glucose by co-TLC and comparison of specific rotation with an authentic sample (R = 0.45, CHCl3–MeOH–H2O, 1:1:0.1, v/v/v, positive value for specific rotation).

Cell Culture

HepG2 cells purchased from American Type Culture Collection (Manassas, VA, USA) were maintained in a high-glucose (4.5 g/L) Dulbecco’s modified Eagle’s medium culture medium supplemented with 10% FBS, 2 mM glutamine, 1000 U/L penicillin, and 100 mg/L streptomycin at 37 °C, 5% CO2.

Glucose Consumption Assay

Cells were detached from the culture flask with a solution of 0.25% trypsin and 1 mM EDTA. Trypsin digestion was stopped by the complete culture medium. The cells were seeded into a 96-well plate at a density 4.0 × 104 cells/well and cultured for 8 h. The cells were incubated with the low-glucose (1 mg/L) detection medium supplemented with 2 mM glutamine and 1% FBS. After overnight incubation in the detection media, the cells were treated for 7 h with metformin (1 mM) or the isolates (at 25 μM; stock solutions made in DMSO) diluted in the detection medium. The glucose concentration in the medium was determined by a glucose assay kit (Eton Bioscience) as per the manufacturer’s instructions. Absorbance was measured at 490 nm, and the assay was performed using 3 replicates per test sample.

Determination of p-AMPK by Western Blot

The cells were seeded into a 6 well plate for 8 h followed by overnight incubation in low glucose media. After 24 h treatment with metformin (1 mM) or the test compounds 1 and 6 (at 25 and 100 μM), total proteins were isolated using RIPA buffer and quantified by the bichonic assay (Pierce, Rockford, IL). Protein homogenates (20 μg/lane) were electrophoretically separated by 8% SDS-PAGE and then transblotted onto Immobilion PVDF membrane (Millipore EMD Corporation, Billerica, MA). Membranes were blocked in 5% non-fat dry milk followed by incubation with primary antibodies p-AMPK, AMPK and β-actin (Cell Signaling Technologies, Danvers, MA) overnight. Membranes were washed 3 times with tris-buffered saline with 0.1% Tween 20 (TBST) followed by incubation with respective secondary horse radish peroxidase-conjugated antibodies (Sigma-Aldrich, St. Louis, MO) for 1 h. After washing the membranes 3 times with TBST, bands were detected on X-ray films using an ECL chemiluminescence detection kit (GE Healthcare, Piscataway, NJ) according to the manufacturer’s protocol..